Silicon coated graphite



Si Deposition Rate (mg/cm min.)-

2 i e x9 March 18, 1969 1 KALNlN 3,433,682

SILICON COATED GRAPHITE Filed July 6, 1965 0 I500 I400 I300 I200 H00I000 I I I I I I .9

Ea =45 KCAL/MOL INVENTOR.

Ilmur L. KoInin ATTORNEY United States Patent 3,433,682 SILICON COATEDGRAPHITE Ilmar L. Kalnin, Millington, N.J., assignor to AmericanStandard Inc., New York, N.Y., a corporation of Delaware I Filed July 6,1965, Ser. No. 469,377 US. Cl. 148--6.3 13 Claims Int. Cl. C23f 7/00ABSTRACT OF THE DISCLOSURE A process for coating graphite with siliconin which a preliminary coating is made of a material capable of forminga eutectic alloy with silicon and then the silicon coating is depositedat a temperature above the melting point of the eutectic alloy.

The present invention is directed to improvements in the production ofcoatings on articles having difficult-towet or irregular or defectivesurfaces. More particularly, the invention is concerned with an improvedmethodfor depositing a protective coating on a porous polycrystallinebase article, such as one composed of graphite.

It is known that the deposition of a coating from the vapor phase onto aforeign polycrystalline body often produces non-uniform and porousdeposits. Thus, the vapor phase deposition of silicon on graphite rodsand mold inserts at constant temperature and flow increases up to acertain limit with an increase in the SiCl concentration in the carriergas. To increase the deposition rate it is the practice to increase theSiCl concentration as much as possible. Initially, the rate ofpenetration of the silicon into the graphite is mainly dependent on thetemperature and the porosity of the graphite. If the deposition rate ofthe silicon does not approximately equal the rate of its penetration,the deposited silicon will accumulate on the graphite surface in theform of peaked protrusions. Such a coating cannot be utilized withoutfurther machining or grinding which addition-a1 operations addconsiderably to the cost of production and lead to a high percentage ofrejects owing to breakage during these operations.

One prior art attempt to remedy the above-outlined problem has consistedin continuing the penetration of the silicon after the depositionprocess has been stopped. This technique, however, is not consideredsatisfactory as it often leads to non-uniform penetration. Furthermore,since this penetration is a slow process, an excessively long time wouldbe required for the smoothing of the accumulated surface deposits.

With a view to overcoming the above-outlined deficiencies of the priorart techniques, themain object of this invention is to provide uniformprotective coatings on graphite and similar polycrystalline articles.

Another object of this invention is to provide an improved method forcoating graphite and similar materials.

A further object of this invention is to increase the formation rate ofvapor-deposited coatings.

Yet, another important object of the invention is to provide a methodfor applying a protective coating on porous, uneven or otherwisedefective substrates.

These and other related objects, .features and advantages of .thepresent invention will be more readily understood as the descriptionthereof proceeds, particularly when taken together with the accompanyingdrawing wherein:

The figure is a hypothetical phase diagram showing the phase changesthat take place when the silicon is vapor 3,433,682 Patented Mar. 18,1969 roe ' microns of a metal or other material which forms an alloywith the material to be deposited at a temperature below the depositiontemperature but which has little or no solubility in the base materialwhen heated at the deposition temperature. The precoated material isthen coated at a constant deposition temperature, high enough to permitthe diffusion of the coating material into the precoat. As a result,alloying takes place and a liquid is formed on the base material for aperiod of time, dependingon the thickness of the precoat. The liquidsurface covers the nucleating centers of the original base substrate andtakes over as the new substrate for further growth. As the depositionand the diffusion proceed, the liquidus resolidifies and provides auniform base for the further deposition of a smooth coating to thedesired thickness. In addition, the precoat causes a change in thedeposition kinetics resulting in faster deposition at lowertemperaturethan otherwise.

In the practice of the invention it is important that the precoatmaterial does not react withthe substrate nor evaporate readily.Similarly, the precoat and the protective coating to be deposited shouldhave a eutectic or at least a peritectic point which lies below thedeposition temperature. The deposition temperature which is em ployedshould be high enough to permit rapid alloying by diffusion between theprecoat and the coating. During the depositionthe liquidus should wetthe base material and should not ball up into droplets.

Among suitable precoating materials for graphite are gold and nickelwhich form a eutectic melt with the silicon below the depositiontemperature between the deposited silicon and the original graphitesubstrate. The precoating can be applied by electroplating, painting,spraying or otherwise covering the substrate. The deposition of siliconfrom SiCl then proceeds in accordance with the principles showing thehypothetical phase diagram appearing on FIG. 4. Other materialsequivalent to gold and nickel for the purposes of the claimed inventioninclude for example, but without limitation, manganese, iron, cobalt,palladium and platinum.

As indicated in the phase diagram, as the silicon de= posits itdissolves in the precoating material (T Ef s T forming a mixture whichmelts when its composition has reached" the liquidus temperature at T Trefers to the temperature at which silicon is deposited and the cymbol Trefers to that temperature at which the pre coat material melts. In themolten state, the initial number of the active nucleation and growthcenters, formed on the original surface, are eliminated. The depositionthen proceeds for a time at the surface of the melt but as the meltbecomes richer in vapor-deposited silicon it re-solidifies and from thenon it is the pure silicon which deposits on the new solid surface whichacts as a new 'shows a fourfold decrease in the activation energy. The

deposition rates for similar precoated and uncoated graphite substratesare compared in Table 1 below.

TABLE I [The Si deposition rate constants for nickel precoated anduncoated AUG graphite substrates, k and k... The symbol k refers to thedeposition rate constant of silicon onto a precoat substrate while thesymbol K refers to the deposition rate constant of silicon into anuncoated graphite substrate] The following examples illustrate thepractice of the claimed invention.

EXAMPLE 1 A cylinder of AUC grade graphite having a diameter of 19.3 mm.and a height of 51.2 mm. weighing 24.54235 g. precoated with gold asfollows. The specimen was washed with distilled water, degreased withacetone sol vent and immersed in a gold cyanide plating bath of thewell-known composition: potassium gold cyanide 8 g./l., potassiumcyanide 4 g./l., potassium carbonate 8 g./l., potassium hydrogenphosphate 8 g./l. and electro plated at 60 C. and current density ofamp/ft. for 15 minutes. The cylinder weight after electroplating was25.11232 g. indicating the weight gain of gold as 659.97 mg.

After precoating the cylinder was inserted in a quartz reaction chamber.Hydrogen gas and a nitrogen gas purge were passed over the cylinder. Thereaction chamber was heated with a radio frequency heat source to 1300C. for minutes. Then silicon tetrachloride gas in a nitrogen gas carrierwas passed through the chamber at atmospheric pressure at 1300" C. for60 minutes. The cylinder was weighed and found to have increased inweight by 0.40368 g. The appearance of its external surfaces wasexcellent as is apparent from FIG. 2.

EXAMPLE 2 A cylinder similar to that used in Example 1 and weighing24.27353 g. was coated with nickel by electroplating from a conventionalaqueous electroplating bath containing 200 g./l. of nickel sulphate, 50g./l. of nickel chloride and 30 g./l. of boric acid for five minutesunder an amperage of 10 amp/ft. and solution temperature of 40 C. Thecylinder was weighed after electroplating and had picked up 0.18121 g.The cylinder was then preheated at 700 C. for 5 minutes under vacuum andthen at 1000 C. for another 5 minutes. It was then coated with siliconin a quartz reaction chamber at 1000 C. using SiCL; in a nitrogencarrier for 60 minutes. The coated cylinder was Weighed and found tohave picked up 430 mg. in weight. The external appearance of thecylinder was free from flaws.

EXAMPLE 3 A cylinder similar to that used in Example 1 and weighing24.40151 g. was coated with nickel by electroplating for 5 minutes underan amperage of 1 amp. at a solution temperature of 40 C. The cylinderpicked up 0.54886 g. The cylinder was then preheated under vacuum at 700C. for 5 minutes and then at 1000" C. for 5 minutes. Nitrogen wasintroduced in the reaction chamber then hydrogen 'and the temperaturewas raised from 1000 C. to 1200" C. as the silicon tetrachloride gas wasintroduced and passed through for 60 minutes until 0.5489 g. of siliconhad been picked up. The coated sample presented a uniform surface as isapparent from FIG. 3 of the drawing.

EXAMPLE 4 A cylinder similar to that used in the preceding exampies andweighing 24.4000 g. was pre-coated with nickel 4 as in Example 3 untilit had picked up 0.46522 g. of nickel. It was then siliconized as inExample 3 'but at 1400 C. until it had picked up 465.2 mg of silicon.The external surfaces of the cylinder were uniform.

EXAMPLE 5 An AUC graphite rod was precoated with nickel as in Example 3until it had picked up 0.43358 gram. The rod was then siliconized as inthe above examples at 1300 C. until it had picked up 433.6 mg. ofsilicon. The external surfaces of the rod were all uniformly coated.

EXAMPLE 6 A rod similar to that used in Example 5 was processed as inthat example except that it was siliconized at 1500 C. for 36 minutes,until it had picked up 266.8 mg. of silicon. It was noted that itsexternal surfaces were all uniform.

EXAMPLE 7 Pre-coat applied by evaporation (a) A cylindrical graphitedisc, weighing 3.201 g. was placed in a suitable position in a Veecovacuum evaporator. Gold was then deposited on it from a tungsten spiralboat containing a pure gold charge. The evaporation was done in a vacuumof 10* mm. mercury at 400 watts heating power for 30 minutes. The weightafter evaporation was 3.215 g. indicating 14 mg. of gold deposited onapproximately 5 cm. of graphite surface which corresponds to a goldlayer approximately 1.5 microns thick.

(b) The same process is repeated using another graphite disc and purenickel charge as the source of vapor. The evaporation was done at 2X 10-mm. mercury at 300 watts heating power for 40 minutes. The weight of thespecimen was 3.528 g. before and 3.540 g. after the coating, The weightgain, 12 mg., of the very uniform Ni coating correspondings toapproximately 3 micron thickness.

EXAMPLE 8 Precoat applied by brushing A commercially obtainable goldpaste, containing approximately 20% by weight of powdered gold suspendedin a viscous liquid binder of an organic nature (Paste Gold UR-Ol-FM)was applied uniformly to the surface of a clean graphite disc by meansof a stiff brush. The specimen was then oven-fired at600 C. for 30minutes in a carbon dioxide atmosphere to burnoff the organic binderleaving a uniform gold coat behind. The weight of the graphite specimenbefore the application was 2.90081 g., weight after firing: 2.96195 g.,giving a gold layer of 61.14 mg. in weight or approximately 7 micronsthick.

EXAMPLE 9 Precoat applied by spraying A commercial proprietary solution,Nickel Resinate, No. 58-A, made by the Engelhard Industries, Inc., Newark, N.J., and containing dissolved nickel was sprayed by means of aconventional atomizing gun, such as manufactured by the Paasche AirbrushCo., Chicago, Ill., onto a clean graphite surface heated at 200 C. torapidly dry and thus immobilize the sprayed liquid particles. Thedeposit was then heated at 600 C. in a slightly oxidizing CO atmosphereto burn off the organic component and leaving behind a layer of nickeloxide, NiO, which is then reduced to nickel by heating at 300" C. in aclosed, hy drogen-containing atmosphere, e.g., forming gas N- 10% HWeight before coating: 2.760 mg., after the coating and the reduction:2.806 mg., weight of the Ni layer: 46.1 mg. equivalent to a precoatapproximately 10 microns thick.

EXAMPLE 10 Precoat applied by vapor plating A hydrogen gas stream issaturated with nickel carbonyl, Ni(CO) vapor by passage through a trapcontaining liquid Ni(CO) at room temperature. The saturated hydrogen gasthen is passed through a chamber containing the graphite speciman heatedat or about 200 C. The Ni(CO) decomposes at the hot surface leaving abright uniform coat on thegraphite. The depositon rate is approximately4 mg. of Ni per minute per 1 cm. of area.

The beneficial effects of the invention on the silicon coatings? appliedto graphite articles can be readily appreciated from an examination ofsilicon-silicon carbide coated articles. 1

It will be readily apparent that the silicon carbide coated graphitearticles which are not precoated, are marred by numbers of protrusionsand an uneven surface. By way of contrast, articles which are precoatedin accordance with the present invention exhibit a smooth surface on thearticles.

As shown in Table 1, the silicon deposition rate is increased by usingthe precoating technique of the invention. Unexpectedly, the silicondeposition rate for precoated articles is higher by a factor of 6 at1000 C. as contrasted with the rate for articles which were notprecoated. While this rate difference is most striking at 1000" C., itis likewise evident at all the deposition temperatures investigated.

What is claimed is:

1. A process for forming a silicon coating on graphite comprising thesteps of precoating a graphite base with a thin layer of a materialcapable of forming a eutectic alloy with silicon to form a precoatedgraphite base, and then coating the precoated graphite base with siliconat a temperature above the melting point of said eutectic alloy, saidmaterial being substantially insoluble in graphite at the depositiontemperature of the silicon.

2. The process of claim 1 wherein said material comprises a metalselected from the group consisting of gold, platinum, palladium, nickel,manganese, iron, and cobalt.

3. The process of claim 1 wherein said thin layer of material has athickness of between about 0.1 and 100 microns.

4; The process of claim 1, wherein said precoat is applied to saidsubstrate by electroplating.

5. The process of claim 1, wherein said precoat is applied to saidsubstrate by evaporation under vacuum.

6. The process of claim 5 wherein said precoat material is gold.

7. The process of claim 1, wherein said precoat is applied to saidsubstrate by brushing.

8. The process of claim 7 wherein said precoat mate= rial comprisesabout 20 weight percent of gold suspended in a viscous organic binderand the precoated graphite base is heated in an oxidizing atmosphere ata temperature of about 600 C. for atime sufiicient to remove said binderand leave thereon a precoat of gold.

9. The process of claim 1, wherein said precoat is applied to saidsubstrate by spraying.

- 10. The process of claim 9 wherein said precoat material comprises asolution containing dissolved nickel and wherein said graphite base ispreheated to about 200 C. and said solution is sprayed onto thepreheated graphite base and then the article is heated in an oxidizingatmosphere at about 600 C. to form a layer ofnickel oxide on saidarticle and then said article is heated at about 300 C. in ahydrogen-containing atmosphere to reduce said nickel oxide to nickel.

11. The process of claim 1, wherein said precoat is applied to saidsubstrate by vapor plating.

12. The process of claim 11 wherein said precoat is nickel which isapplied by passing a hydrogen gas stream saturated with nickel carbonylover a graphite article heated to about 200 C.

13. The product formed by the process of claim 1.

References Cited ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

US. Cl. X.R.

